1. Field of the Invention
The present invention generally relates to a thin-film forming apparatus using plasma CVD method and a method for forming thin films; and particularly to a thin-film forming apparatus and method which form thin films using a surface-treated shower plate.
2. Description of the Related Art
The plasma CVD method has been typically used as a method for forming a thin film on a semiconductor substrate. FIG. 1 is a schematic view of a conventional plasma CVD deposition apparatus 1. The plasma CVD apparatus 1 comprises a reaction chamber 6, a susceptor 3 for placing a semiconductor substrate 4 inside the reaction chamber, a showerhead 9 provided over against and practically parallel to the susceptor 3 inside the reaction chamber, and an exhaust port 10 provided at the bottom of the reaction chamber. The susceptor 3 is supported by a heater 2 which heats the substrate 4 to a given temperature (150-450° C.) and is electrically grounded 12 so as to form one side of electrodes for plasma discharge. The showerhead 9 has a hollow structure in which a circular shower plate 11 having a number of fine pores is installed in a cylindrical body 13; a gas inlet port 5 for introducing a reaction gas is provided at the top of the showerhead 9. The showerhead 9 and the gas inlet port 5 are insulated from the reaction chamber 6, are connected with radio-frequency sources 7, 8 and form the other side of the electrodes. Such conventional plasma CVD apparatuses have been universally known, for example, as described in Japanese Patent Laid-open No. 1986-260623, Japanese Patent Laid-open No. 1999-251094, and Japanese Patent Laid-open No. 2001-240973.
A film type and film characteristics to be formed on a surface of the semiconductor substrate 4 change depending on the type and the flow rate of a reaction gas, the substrate temperature, the shower plate temperature, the type of RF frequency, and the spatial distribution and electric potential distribution of plasma. In order to remove a film formed on places other than the substrate, a cleaning process is executed in the conventional plasma CVD apparatus after a consecutive deposition process for a given number of substrates has been completed. According to circumstances, a cleaning process may be executed whenever a deposition process for one piece of substrate is completed.
As described above, the conventional showerhead 9 has a hollow structure and a jet of reaction gas is emitted uniformly from the fine pores (not shown) in the shower plate 11. In order to facilitate maintenance and to reduce component costs, the showerhead 9 has a structure where the shower plate 11 is replaceable. Metal (aluminum or aluminum alloy) surfaces to be connected are exposed on both the shower plate 11 side and the body 13 side, and the two sides are then attached firmly and fixed by bolts, etc.
Conventionally, the surfaces of the body and the shower plate are in metal-to-metal contact in order to prevent interference with passage of electric current and propagation of radio-frequency waves. Further, no special protection is believed to be required because the surfaces are not fully exposed to fluorinated radicals. This is true even when a surface of the shower plate is anodized. FIG. 1(B) is an enlarged view of an integrated portion 21 of the body 13 and the shower plate 11. A surface of the conventional shower plate including pores 17 is coated with an anodic oxide layer 30, except for a peripheral surface 15 which is in metal-to-metal contact with a peripheral surface 14 of the body. It is commonly known in the art that it is needless to say that the peripheral surface 15 is exposed.